The long-range goal of our research is to understand the mechanisms involved in the control of hormone secretion from the magnocellular neuroendocrine cells (MNCs) whose cell bodies reside predominantly in the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus. Since the patterns of electrical activity are dominant factors in the process of neurosecretion by these cells, our immediate goals for the research proposed here are to investigate some of the factors which may influence or determine their activity patterns. In particular we wish to focus on investigating membrane properties, synaptic inputs and non-synaptic interactions, all of which probably play roles in controlling or modulating the process of neurosecretion in this system. In order to accomplish these objectives, we will record intracellular from PVN and perifornical neurons and from both neurons and glial cells in the area of the SON in slices of rat hypothalamus. In some instances the electrodes will be filled with the fluorescent dye, Lucifer Yellow (LY), and in others with horseradish peroxidase (HRP) in order to identify the recorded cell and trace its processes. Cells marked with LY will undergo either subsequent immunocytochemical analysis of hormone content (oxytocin, vasopressin or enkephalin) or electron microscopic (EM) analysis to demonstrate gap junctions after rendering the LY electron dense. HRP-injected cells and their processes will be analyzed at both the light and EM levels in order to determine the collateral branching and axonal terminations of these neurons. Studies of putative transmitters will also be carried out. Thus, we will be able to determine the physiological, morphological and hormonal characteristics of cells in this brain area, as well as to learn more about the local circuitry in which these cell groups apparently participate.